Liquid crystal display technology has become prominent in recent years due primarily to the popularity of digital displays for a variety of information display functions. Of particular note are the use of such displays in hand-held calculators and digital watches.
While there are several types of liquid crystal displays in use, a particularly advantageous display for certain high performance application is known as the "phase-change" display. This type of display has been described in the literature, e.g. D. L. White and G. N. Taylor, "New Absorptive Mode Reflective Liquid Crystal Display Device", Journal of Applied Physics, Vol. 45, pp. 4718-4723, (1974). Briefly, these devices comprise a typical transmissive or reflective liquid crystal display cell having appropriate front and back electrode patterns and features a homogeneously or homeotropically oriented liquid crystal layer comprising host positive nematic liquid crystal, a guest dichroic dye, and an optically active additive in amounts sufficient to provide a cholesteric liquid crystal phase. In the absence of an electric field across the display electrodes, the dichroic dye is oriented to absorb a substantial amount, e.g. 95%, of the unpolarized incident light and the display area exhibits a color characteristic of the dichroic dye. When an electric field is applied to the desired electrode, the liquid crystal layer in register with the electrode is caused to change from a cholesteric phase to a nematic phase in homeotropic alignment due to the positive dielectric anisotropy of the liquid crystal host material. In this state, the dichroic dye in the liquid crystal layer is oriented to absorb relatively little incident light and a "clear" area, corresponding to the electrode area on a colored background, is observed. By selective activation of the electrodes, information can be readily displayed.
Due to its operating mechanism, the phase change display does not require the use of auxiliary polarizers as do other well known types of displays, e.g. twist-nematic and guest-host nematic displays. However, the phase-change displays have a number of inherent disadvantages. One significant negative performance characteristic is the so-called "storage effect" or "after-image scattering" observed when the display is switched from a field-on to a field-off state. Following this transition, the image previously displayed does not immediately disappear, but remains for several seconds or more as a milky pattern which scatters light and may render the displayed information ambiguous, particularly where rapid switching such as in multiplexing, is required. Further, significant amounts, e.g. 5-15% by weight, of the optically active and dichroic additives must be incorporated into the liquid crystal mixture. These "foreign" additives are typically chemically and/or photochemically unstable in liquid crystal display systems and their addition introduces a potential source of harmful decomposition products into the display. In addition, all the optically-active additives of the prior art are nematic liquid crystals and it is well known that liquid crystal systems comprising chemically dissimilar mixtures of liquid crystals have properties which vary nonlinearly and unpredictably with composition and temperature. Thus, typical phase-change liquid crystal mixtures may have properties such as optical anisotropy, phase transition temperature, and electric, magnetic and elastic properties which vary unpredictably.